Shamel Fahmi, Michele Focchi, Andreea Radulescu, Geoff Fink, Victor Barasuol and Claudio Semini
Dynamic Legged Systems (DLS) lab, Istituto Italiano di Tecnologia (IIT)
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STANCE
Locomotion Adaptation over Soft Terrain
ICRA 2020
![](https://s3.amazonaws.com/media-p.slid.es/uploads/1171535/images/6917088/AdobeStock_119482893.jpeg)
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taken from Andrea del Prete, 2019
Shamel Fahmi
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Most WBCs are not terrain aware. They fail to generalize beyond rigid terrain.
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Soft terrain locomotion by itself is difficult because of the induced contact dynamics
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Mismatch between what the WBC assumes vs. what is actually happening
Shamel Fahmi
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Drop the assumption of the rigid terrain. Use a more generic model that accounts for the terrain impedance
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But that’s not enough, how do you know the impedance parameters of the terrain.
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Hmm, estimate it?
Shamel Fahmi
Shamel Fahmi
Soft Terrain Adaptation aNd Compliance Estimation
(STANCE)
Shamel Fahmi
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The Standard WBC (sWBC) framework:
- Locomotion Planner
- State Estimator
- WBC
- Low-level Control
![](https://s3.amazonaws.com/media-p.slid.es/uploads/1171535/images/7356140/all.png)
Shamel Fahmi
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Execute the planned trajectories of the body and swinging leg(s) (a.k.a control tasks)
Goals:
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Keep the robot balanced
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Consistent contact forces and accelerations with the dynamics
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Respect the robot's dynamics and actuation limits
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Respect the contact interaction constraints
Maintain contact consistency
1. Control Tasks Tracking
2. Physical Consistency
3. Joint & Torque Limits
4. Friction Constraints
5. Unilaterality Constraints
6. Stance Task
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Casted as an optimization problem via Quadratic Programming (QP)
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Mapped using inverse dynamics
![](https://s3.amazonaws.com/media-p.slid.es/uploads/1171535/images/7356140/all.png)
Shamel Fahmi
6. Stance Task
5. Unilaterality Constraints
4. Friction Constraints
3. Joint & Torque Limits
2. Physical Consistency
1. Control Tasks Tracking
Goals:
Shamel Fahmi
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Using the KV model
![](https://s3.amazonaws.com/media-p.slid.es/uploads/1171535/images/6917239/pasted-from-clipboard.png)
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The stance task does not hold anymore
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Compliant contact consistent (c3)
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Soft contact dynamics must be taken into account
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c3 Stance Task
Shamel Fahmi
Shamel Fahmi
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Estimate the terrain parameters online
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Based on the current measurements (states)
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Decoupled from the C3WBC but uses the same contact model
![](https://s3.amazonaws.com/media-p.slid.es/uploads/1171535/images/7356140/all.png)
Shamel Fahmi
Shamel Fahmi
Shamel Fahmi
![](https://s3.amazonaws.com/media-p.slid.es/uploads/1171535/images/7373657/g28460.png)
Shamel Fahmi
![](https://s3.amazonaws.com/media-p.slid.es/uploads/1171535/images/7373637/Screenshot_from_2020-05-18_11-17-33.png)
Shamel Fahmi
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Unlike previous work on WBC, we do not assume that the ground is rigid.
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Adapt online to any type of terrain compliance
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C3WBC
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Extends the sWBC but incorporating soft contact dynamics
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C3WBC can adapt to the given terrain compliance
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TCE
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Estimate the terrain compliance online
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Not computationally expensive
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Shamel Fahmi
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With STANCE, HyQ can traverse and transition between multiple terrains with different compliance without pre-tuning.
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STANCE makes the locomotion strategy c3. It allowed HyQ to remain consistent with ground, and thus more robust in challenging and scenarios.
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Each leg independently senses and adapts to changes in terrain compliance.
![](https://s3.amazonaws.com/media-p.slid.es/uploads/1171535/images/7362685/shamel_low.jpg)
![](https://s3.amazonaws.com/media-p.slid.es/uploads/1171535/images/7362684/victor_crop.jpg)
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![](https://s3.amazonaws.com/media-p.slid.es/uploads/1171535/images/7371545/ICRA_2020_logo-flat-hd.png)
HyQ Real pulling a plane
![](https://s3.amazonaws.com/media-p.slid.es/uploads/1171535/images/7382859/f.png)
G. Urbain et. al., "Stance Control Inspired by Cerebellum Stabilizes Reflex-Based Locomotion on HyQ Robot"
A. Bratta et. al., ''On the Hardware Feasibility of Nonlinear Trajectory Optimization for Legged Locomotion based on a Simplified Dynamics''
O. Villarreal et. al., ''MPC-based Controller with Terrain Insight for Dynamic Legged Locomotion''